1. Field
This invention relates to sugar extraction processes. It is particularly directed to the clarification of raw juice extracted from agricultural sources, such as sugar beets, prior to purification of the sucrose contained in that juice.
2. State of the Art
In the conventional production of crystallized sucrose (sugar), a "raw juice" is initially obtained by diffusion of soluble material from beets, cane or other sources. The raw juice is then partially purified. The purpose of this initial purification step is to remove a significant portion of the "nonsucrose" fraction from the juice. The partially purified juice exhibits improved subsequent processing, yields a higher recovery of crystallized product and improves product quality with respect to color, odor, taste and solution turbidity. As applied to sugar beets, raw beet juice is usually obtained as a result of countercurrent extraction of sliced beets with hot water. This process results in a high load of suspended solids, typically, 3-4 volume percent.
The most commonly used method for raw beet juice purification is ubiquitous, and is based upon the addition of lime and carbon dioxide. The initial steps of this method occur prior to crystallization, during a phase commonly referred to as the "beet end" of the process. The sugar beets are typically diffused with hot water to extract a "raw juice" or "diffusion juice". The raw juice contains (1) sucrose (2) nonsucroses and (3) water. The term "nonsucroses" includes all of the sugar beet-derived substances, including both dissolved and undissolved solids, other than sucrose, in the juice. Other constituents which may be present in the raw juice are not of concern to the present invention.
The raw juice is heated to high temperature, and a solution/suspension of calcium oxide and water (milk of lime) is added to the juice. The juice is then treated with carbon dioxide gas to precipitate the calcium oxide as calcium carbonate. This step is commonly called "first carbonation," and it is the foundation of the conventional purification scheme, resulting in a "first carbonation juice." During this step, various nonsucrose compounds, color etc. are removed or transformed by reaction with the lime or by absorption by the calcium carbonate precipitate.
Conventionally, the calcium oxide and the carbon dioxide are produced by heating limerock (calcium carbonate) in a high temperature kiln. The calcium carbonate decomposes to calcium oxide and carbon dioxide, which are then recombined in the first carbonation step. The resulting calcium carbonate "mud" is usually removed from the first carbonation juice by settling clarifiers or by appropriate filters. The resulting "lime waste" is difficult to dispose of and contains about 20 percent to 30 percent of the original raw juice non sucrose. The first carbonation juice is most commonly sent to a second carbon dioxide gassing tank (without lime addition). This gassing step is often referred to as "second carbonation." The purpose of the second carbonation step is to reduce the level of calcium present in the treated ("second carbonation") juice by precipitating the calcium ions as insoluble calcium carbonate. The calcium precipitates, often called "limesalts," can form a noxious scale in downstream equipment, such as evaporators. The second carbonation juice is usually filtered to remove the precipitated calcium carbonate.
In conventional processes, liming and carbonation are used to coagulate and chemically react with dissolved non-sugar components. Due to high suspended solids load, lime is often used excessively to provide enough calcium carbonate which serves as incompressible filter-aid in subsequent filtration. Thus, additional suspended solids load generally results in excess amounts of calcium carbonate waste. Production of lime and disposal of waste product create environmental problems, such as high carbon monoxide emissions, water contamination and the creation of odors related to decomposition of organic matter.
Various methods and equipment used for purifying raw sugar juice by ion exchange are disclosed in British Patent No. 1,043,102; U.S. Pat. Nos. 3,618,589; 3,785,863; 4,140,541; and 4,331,483. A proposed method of purification of raw sugar juice involving membrane ultrafiltration is disclosed in U.S. Pat. No. 4,432,806. A method and apparatus for chromatographic molasses separation are disclosed in U.S. Pat. No. 4,312,678. Other methods and apparatus using simulated moving bed chromatographic separators are disclosed in U.S. Pat. Nos. 2,985,589; 4,182,633; 4,412,866; and 5,102,553.
Juice subjected to conventional clarification is not easily purified by methods such as membrane filtration, ion-exchange, multimedia filtration, chromatography and other methods requiring relatively low suspended solids load. Juice treated with lime also has a relatively high hardness level which makes it difficult to treat directly in highly efficient separation methods such as chromatography.
Chemical treatment of juice has been proposed (U.S. Pat. No. 4,432,806) with prior mechanical separation of undissolved components. Low molecular weight non-sugars are converted to high molecular weight non-sugars and subsequently separated from sucrose by ultrafiltration, thereby enhancing sucrose purity. Mechanical removal of suspended solids is a difficult task to accomplish, however.
U.S. Pat. No. 5,544,227 discloses a procedure by which raw beet or cane juice is heated to 70-105.degree. C. and vigorously mixed with a cationic flocculating agent prior to its introduction to a clarifier. Part of the flocculated suspended solids is settled in the clarifier. The clarifier overflow stream is fed to a membrane filtration unit where the rest of the colloidal material and suspended solids are removed. However, addition of a flocculent may adversely affect membrane performance. Moreover, heating of the juice results in significant losses of sucrose, due to inversion.
Commonly assigned U.S. Pat. No. 5,466,294 discloses a sugar beet juice purification process in which the traditional liming and carbonation purification procedures are replaced with ion exchange softening and chromatographic separation operations. The disclosure of the '294 patent is incorporated by reference as a part of this disclosure for its teachings concerning the state of the art in purifying diffusion juices generally. A description of conventional clarification technology, as applied to sugar beets, may be found in the book authored by R. A. McGinnis, "Beet Sugar Technology", Beet Sugar Development Foundation, Ft. Collins, Colo., (3rd Ed, 1982).